About 100 people have joined the Islamic State militant group from countries in the Caribbean and South America, and existing human smuggling networks are in place that could allow them to infiltrate the United States if they return, said a top U.S. general on Thursday.

Marine Gen. John F. Kelly, chief of U.S. Southern Command, told the Senate Armed Services Committee that the networks are “so efficient that if a terrorist or almost anyone wants to get into our country, they just pay the fare.”

“No one checks their passports,” the general said. “No one, you know, they don’t go through metal detectors. No one cares why they’re coming. They just ride this network.

More than 15,000 foreign fighters have poured into the Middle East to join the Islamic State, traveling primarily to Syria. Kelly, who oversees U.S. military operations in Central and South America and the Caribbean, acknowledged that the number of fighters coming from his region is small, but said that the countries involved don’t have the ability to monitor those returning well.

“I would suspect … that while they’re in Syria they’ll get good at killing and they’ll pick up some real job skills in terms of explosives and beheadings and things like that,” Kelly said. “Everyone is concerned, of course, if they come home because if they go over radicalized, one can expect that they’ll come back at least that radicalized, but with really good job skills.”

Foreign fighters have gone to Syria from Jamaica and Trinidad and Tobago in the Caribbean and Suriname and Venezuela in South America, Kelly said. The freedom of movement in Central and South America, he said, makes it easy for people to move from those countries to the north over the Isthmus of Panama and into North America.

“Not to take anything away from the Department of Homeland Security men and women and the FBI and all. They just do a magnificent job,” Kelly said. “But the amount of movement… and the sophistication of the network overwhelms our ability to stop everything. So I think if [foreign fighters] get back to some of these countries that I’ve described, it’s pretty easy for them to move around.”